1. Field of the Invention
This invention relates to gas turbine engines and more specifically to the diffuser which is located between the combustion and the compression sections of axial flow engines.
2. Description of the Prior Art
An axial flow gas turbine engine principally comprises a compression section, a combustion section and a turbine section. Gases compressed within the compression section are flowed axially downstream to the combustion section where a port ion of the working medium gases is mixed with fuel in a combustion chamber to form a combustable mixture which is burned to increase the kinetic energy of the flowing gases.
A diffuser is interposed in the flow path of medium gases between the compression and the combustion sections to decrease the velocity and increase the static pressure of the flowing gases. Low velocity gases are preferred within the combustion section to prevent the accumulation of substantial flow losses as the medium is driven through the various passages and apertures within the combustion section.
An axial flow engine having a conventional diffuser section is shown in U.S. Pat. No. 2,686,401 to Newcomb. As is shown in FIG. 1 of Newcomb a diverging annular passage extends between the compression and the combustion section and forms a gas diffuser having a flow path area which increases gradually in the downstream direction from the compression section to the combustion section. In order to fully recover the static pressure of the flowing gases in a diffusion process the length of the diffusing element must be substantial. Increasing the angle of divergence between the diffuser walls increases the rate of diffusion. However, when the angle becomes excessive, the flow separates from the walls and substantial flow losses result. U.S. Pat. No. 3,300,121 to Johnson shows a diffuser section having rapidly diverging walls to shorten the axial length of the diffuser section. In Johnson additional upstream apparatus is provided for flowing compressed gases to the region of separation to reduce the flow losses.
The static pressure of the flowing medium gases is principally recovered during the initial portion of the diffusion process. Accordingly, a diffuser section having a shortened axial length can be provided where the initial diffusion is accomplished in a highly controlled diffusion element and the remaining diffusion accomplished in a diffuser dump region where the static pressure instantaneously increases and the flow velocity instantaneously decreases. An example of such apparatus is shown in U.S. Pat. No. 3,742,706 to Klompas. In all diffusion apparatus it is extremely important that regions of unstable pressure not be generated within the flowing medium gases. Any nonuniform pressure regions will be carried downstream by the flowing medium to the combustion section where they detrimentally effect uniform burning within the combustion chamber which subsequently precipitates uneven temperature zones within the turbine section. Uneven temperature zones limit the life of turbine components and are to be avoided where possible. To eliminate flow instability engine designers strive to provide diffuser sections which have concnetric inner and outer walls. Concentric apparatus which is initially provided must be held in concentric alignment when exposed to the severe thermal environment of th operating engine.
Continuing efforts are underway to provide efficient gas diffusion apparatus having a reduced axial length which is capable of increasing the static pressure and reducing the velocity of working medium gases flowing to the combustion section without inducing instability regions in the flowing medium.